The present invention relates to a process for producing high molecular weight monovinylidene aromatic polymers and polymodal molecular weight monovinylidene aromatic polymer compositions.
High weight average molecular weight (Mw) monovinylidene aromatic polymers have been previously produced by free radical polymerization in the presence of a vinyl acid have a pKa of less than 2 as disclosed in U.S. Pat. No. 5,145,924, which is incorporated herein by reference and in U.S. Pat. Nos. 5,962,605 and 5,948,874. The free radical polymerization is typically conducted in the presence of an initiator. However, it is well known in the art that at some point during the polymerization, the initiator is expended and thermal polymerization occurs unless additional initiator is added. During thermal polymerization of vinyl aromatic monomer, conversion becomes difficult due to the presence of the acid and low Mw fractions are produced, as taught in J. Phys. Org. Chem (1995) 8, p. 301. Additionally, the presence of acid during thermal polymerization catalyzes the formation of 1-phenyltetralin, a by-product, as described in Polymer (1992) 33, p. 3055. At the high temperatures used to increase conversion in the absence of initiator, 1-phenyltetralin forms rapidly in the presence of acid and must be removed from the polymer composition.
Polymodal molecular weight monovinylidene aromatic polymer compositions, wherein the polymer composition comprises more than one distinct weight average molecular weight, have previously been produced in a number of ways including blending and in situ polymerization. Blending two polymers having different molecular weights offers a high degree of control, but is inefficient due to the addition of the compounding step. U.S. Pat. No. 4,585,825 discloses a bimodal composition, however, the high molecular weight polymer is produced at low temperatures and low conversion rates which require very long reaction times. U.S. Pat. Nos. 5,962,605, and 5,948,874, herein incorporated by reference, disclose bimodal compositions produced using free radical polymerization, characterized in that high molecular weight polymer is produced in the presence of a vinyl acid having a pKa of less than 2.0, and low molecular weight polymer is produced by adding initiator and chain transfer agent to the reaction mixture containing high molecular weight polymer. However, the presence of the vinyl acid retards the polymerization rate and continues to encourage high molecular weight polymer production, thus requiring large amounts of chain transfer agent in order to produce the low molecular weight polymer.
Therefore, it remains highly desirable to provide an improved process to produce high molecular weight monovinylidene aromatic polymers, which does not require additional initiators, and reduces by-product formation; and to provide an improved process to produce polymodal compositions comprising such high molecular weight polymers which does not require large amounts of chain transfer agent in order to make a low molecular weight polymer.